Boston fire officials say a vicious restaurant fire that claimed the lives of two Boston firefighters and injured 10 others Wednesday night may have been smoldering for at least an hour in a double-layered ceiling before workers and customers noticed it and evacuated.

When fire crews arrived minutes later, conditions inside the building were ripe for a backdraft fire to explode around them, officials said.

The two firefighters killed in the blaze were identified as Paul Cahill, 55, of Scituate, Mass., and Warren Payne, 53, of Canton, Mass. Both worked out of Engine 30, Ladder 25 company, which is located near the restaurant on Centre Street in the city's West Roxbury neighborhood.

Ten other firefighters were taken to hospitals with less serious injuries and a paramedic was evaluated for chest pains. Two were initially listed in critical condition, but their conditions were upgraded. One was later released and another remained hospitalized Thursday afternoon.

Fire officials said the fire broke out about 9 p.m. at the Tai-Ho Mandarin and Cantonese restaurant.

They said a flue carrying grease was venting up through the kitchen roof and some of the grease had accumulated in a crawl space. Once the fire began, toxic and flammable gases also began accumulating in the spaces between the double ceiling.

When a three-ton air conditioner fell through the ceiling, there was enough flame and gases to create backdraft conditions that exploded around the firefighters, officials said.

"I've never seen anything like it. I mean, it was definitely a disaster," witness Brian Strickland said. "First I just started hearing sirens and everything and then, out of nowhere, we saw the flames start coming up."

Seven to 10 of the first firefighters to respond became trapped inside and were unable to escape in the heavy smoke and heat. The second crew to arrive helped to pull them from the burning building.

"They advanced their lines into the store, and they became trapped and they were disoriented and they couldn't find their way out," Fire Chief Kevin MacCurtain said.

The cause of the blaze is under investigation, but MacCurtain initially blamed the huge backdraft created by the collapse of a rooftop air conditioning unit for the fatal nature of the blaze. He said when the three-ton unit fell through the roof it created a hole that acted as a chimney, creating backdraft conditions.

"When it dropped into the void, it created all the oxygen it needed," MacCurtain said. Flames then roared through the building.

Investigators said it did not appear that the air conditioning unit actually collapsed on the men.

"These brave men died in the line of duty protecting the people of Boston and I ask that your prayers be with them," Boston Fire Commissioner Roderick Fraser said. "People have to understand it is a dangerous job and it's just unfortunate that we lost these two men tonight."

Both of the victims were veteran firefighters who worked in the close-knit, predominantly Irish Boston community, and both men had two children. Payne, a father of two, had been with the department for 19 years. Cahill, a father of three, was with the department for 14 years, officials said.

"Everyone's in mourning. Everyone's in shock. The firefighters ... we live together, we eat together, we know each others' families. We're like brothers, and tonight we lost two of our brothers. Everyone's devastated," Boston firefighters' union president Ed Kelly said.

West Roxbury residents stopped by the Centre Street fire station all morning to offer condolences.

"It's a tragedy. It's horrible. Everyone gets up and goes to work and comes home. These guys ... you know..." Mark Scanlon said, his choked voice trailing off. He said he was friends with many of the firefighters who worked at the station.

Scanlon left a flag. Bouquets and cards were also left outside the building.

"It feels like you've lost somebody right in your family, that's what it feels like," nurse Amy White said. She said her father and brother were both firefighters.

"Always in the house was a fire radio. And they take care of us, so we want to take care of them," she said.

Andrea Tapp came to say thank you.

"They're very brave people," she said. "I mean, I wouldn't do it. Would you?"

Boston's Roman Catholic Cardinal Sean O'Malley offered condolences saying, "At this terribly sad moment we pray for Paul and Warren, their families and friends, the more than 100 firefighters who responded at the fire, and all the men and women of the Boston Fire Department. We pray for those injured last night and ask God to bless them with the strength to recover."

Massachusetts Gov. Deval Patrick ordered flags around the state lowered until after the firefighters funerals. Firefighters from Boston's Brighton neighborhood covered the Centre Street station for their fallen colleagues, spending part of the day draping black bunting around the building.

Cahill and Payne were the first two Boston firefighters to die in the line of duty since 1994.

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